As effects of catechins, there have been reported a suppressing effect on an increase in cholesterol level and an inhibitory effect on α-amylase activity. To make such physiological effects activated, it is necessary for an adult to drink 4 to 5 cups of tea in a day. Accordingly, it has been demanded to develop a technique that permits the addition of catechins at a high concentration in beverages to facilitate the ingestion of a large amount of catechins. As one of such methods, catechins are added in a dissolved form to a beverage by using a concentrate, purified product or the like of a tea extract.
However, a beverage containing non-polymer catechins at a high concentration causes strong bitterness and astringency when drunken, and therefore, is not suited for habitual drinking. So far, there has been reported a method which includes adding a high molecular substance such as cyclic oligosaccharide, as a method for reducing bitterness and astringency (JP-B-2566801). However, a beverage containing non-polymer catechins at a high concentration requires a large amount of cyclic oligosaccharide and, if cyclic oligosaccharide is added excessively, the inherent flavor and taste of the beverage could be impaired by its flavor and taste. Further, when stored over a long time after its production a beverage with a concentrate or purified product of a tea extract added therein, develops grouts and precipitates and cause considerable damage to its external appearance. Conventional tea beverages, especially green tea beverages may also develop grouts and precipitates during storage, and in the case of beverages with a concentrate or purified product of a green tea extract added therein, the damage in their external appearance is more critical. The grouts and precipitates in green tea beverages are considered to stem from the formation of complexes from components such as polysaccharides, proteins, polyphenols and metal ions. The mechanisms of formation of grouts and precipitates are complicated problems though a variety of countermeasures have been considered to date. As countermeasures directed to high-molecular components in green tea, for example, there have been reported methods for suppressing the formation of grouts by degrading components, which would otherwise form high molecular complexes, into lower molecular weight substances by enzymatic treatment (JP-A-05-328901, JP-A-11-308965); and a method for suppressing the formation of grouts by fractionating green tea components through an ultra filtration membrane to substantially eliminate high molecular substances whose molecular weights are 10,000 and higher (JP-A-04-045744). As a countermeasure directed to metal ions, on the other hand, there has been reported a method for reducing coloring and turbidity by conducting treatment with a cation exchange resin, followed by nanofiltration (JP-A-11-504224) There has also been reported a method, which includes conducting a cation exchange resin treatment with a cation exchange resin on which potassium ions have been bonded beforehand (JP-A-10-165098).